The present invention relates to a safe electric double layer capacitor for power supplies for starting cell motors or as back-up power supplies for hybrid vehicle power supplies or personal computers, etc.
Electric double layer capacitors, wherein a non-woven fabric of polyethylene or polypropylene or a microporous film is interposed as a separator between two polarized electrodes where an electrode surface is formed on a collector surface using a binder of activated carbon or carbon particles and a polyvinylidene fluoride based polymer or polarized electrodes where aluminum is plasma-deposited on an activated carbon fiber cloth so as to form a conductive layer, and the separator being impregnated with a nonaqueous electrolytic solution, have high output and superior cycle characteristics and thus advances are being made in technology thereof.
When electric double layer capacitors employing nonaqueous electrolytic solution are subjected to high-temperatures, the nonaqueous electrolytes vaporize and the internal pressure of the nonaqueous electrolytes increases so that the shape of the capacitor changes, thereby causing not only changes in the characteristics of the capacitor dramatically but also causing risks of explosions and fire, pausing difficulties with regards to safety management.
A further problem with electric double layer capacitors is that carbon particles produced by such effect of coming apart and the like may move to the opposing electrode when carbon electrodes using activated carbon or carbon particles are employed. Particles produced by the coming apart may then move to the anode side by electromigration and such and thereby causing the leakage current to increase or short-circuits to occur more easily. The migration of particles between electrodes means self-discharge and the ability to prevent migration of particles is substantially related to the fundamental performance of the capacitor.
On the other hand, the degree of difficulty of ionic migration within the electrolytic solution is manifested in the form of liquid resistance, and an increase in this value shows a tendency of difficulty in smooth charging and discharging of the capacitor. It has therefore been necessary to increase the number of holes per unit surface area of the separator in order to reduced the resistance. This can also be achieved by making the separator thin because the fluid resistance can also be lowered by narrowing the distance between the electrodes. Generally, an extremely strongly polarized organic solvent is employed as the electrolyte for an electric double layer capacitor so that a great deal of salt may be dissolved in the solvent, and thus it is necessary that the separator does not react with this solvent or become dissolved by the solvent and also that the electrolyte including the separator must be resistant to mechanical stresses.
Various types of separators have been proposed for such separators. Examples may be found in such as high-density separators referred to as capacitor papers, low-density capacitor paper referred to as electrolytic paper, micro-porous items substance of polyethelene or polypropelene film, and non-woven fabric using polyethelene or polypropelene fibers. However, in the case of capacitor papers there is high inhibition against the conductivity of the electrolyte thereby making it impossible to obtain the desired electrostatic capacitance and internal resistance, and on the other hand in the case of electrolytic papers, the average pore diameter is greater than the size of the carbon particles and carbon particles therefore pass through the papers whereby the papers could not be made into the desired low leakage current capacitors.
Technology for an electric double layer capacitor with the capacitor having increased fluid retention of the nonaqueous electrolytic solution and with good handling characteristics of the nonaqueous electrolyte layer is disclosed in Japanese Patent Laid-open Publication No. Hei 6-36972.
One of the noticeable features of this capacitor is the use as separators of sustances each having 1 to 20% by weight of acrylic resin particles of a particle diameter of 10 to 100 xcexcm. In the case of electric double layer capacitors employing this separator there are recognized improvement in the permeability of nonaqueous electrolytic solution into the separator structure, fluid retention characteristics and the capacitor assembly process, but there still remains difficulty with the capacitors that vaporization of the nonaqueous electrolytic solution can not yet be prevented.
Technology for an electric double layer capacitor employing, as a microporous separator which prevent the passage of carbon particles, bacterial cellulose obtained from an acetic acid bacteria stationary culture is disclosed in Japanese Patent Laid-open Publication No. Hei 9-129509. A major feature of this capacitor is the use of cellulose with a microfilament mesh structure as the separator. However, in case of capacitors employing this separator it is possible to prevent the passage of carbon particles, but there is still difficulty that the evaporation of the aqueous electrolytic solution can not be prevented.
Developments in electric double layer capacitors without these kinds of problems are in progress and there has been used technologies for solidifying nonaqueous electrolytic solution in particular for forming the nonaqueous electrolytic solution into a gel using as gelling agents polymers of polyacrylonitrile or acrylonitrile and (meta) acrylate. The polymer gel electrolyte obtained by heating and dissolving the acrylonitrile based polymer into the nonaqueous electrolytic solution and followed by cooling the same exhibits good ionic conduction, fluid retention and prevention of the passage of carbon particles, but the gel electrolyte shows high tackiness and thereby pausing difficulty in handling, whereby the assembly of electric double layer capacitors using this gel electrolyte faces therefore many difficulties and the manufacture of high-performance electric double layer capacitors becomes extremely difficult.
The inventors carried out investigations with the object of manufacturing in an efficient manner a high-performance electric double layer capacitor by improving the handling characteristics of each part in an assembly step, wherein the nonaqueous electrolytic solution does not evaporate, the prevention of the passage of carbon particles is good, and the capasitor is safe, and wherein the characteristics of impregnation and liquid retention for nonaqueous electrolytic solution are good. The inventors then found from the results that this object can be achieved by using as a carrier of the nonaqueous electrolytic solution a fibrous sheet-like substance comprising mainly a fiber-like substance or pulp-like substance of organic polymer soluble in the nonaqueous electrolytic solution and having gel-forming properties, and thereby completing the present invention. The gist of the invention is directed to an electric double layer capacitor with a polymer gel electrolyte layer interposed between polarizable electrodes, and is characterized in that an electrolyte is used wherein a fibrous sheet, comprising mainly a fiber-like or pulp-like material of an organic polymer which is soluble in or swellable by a nonaquerous electrolytic solution to form a gell, with a nonaquerous electrolytic solution and thereafter a part of the fibrous sheet is dissolved or swollen to give a gel.
The fiber-like or pulp-like material to be used in the practice of the present invention has a shape similar in appearance to a fiber of about 1 to 100 xcexcm in apparent diameter and a length of approximately 5 mm or less may be preferably used. A fibrous sheet-like material formed from a fiber-like or pulp-like material having the apparent diameter which is too small in apparent diameter is not preferable because it shows insufficient strength or insufficient handling characteristics. On the other hand, in the case of a fibrous sheet-like material formed from a fiber-like or pulp-like material which is too large there is manifested low impregnating ability, solubility and swelling properties, and thus it is low in the ability to be formed into a polymer gel electrolyte layer with good performance characteristics. In the case of a fibrous sheet-like material formed from a fiber-like or pulp-like material which is long in a longitudinal direction, it is also difficult to make the fibrous sheet-like material to have an even thickness. It is possible to use a material having a large number of fibril for the fiber-like or pulp-like material used in the present invention, and the fibrous sheet formed from the fiber-like or pulp-like material has the advantage of good mechanical strength and an excellent impregnation characteristic under nonaquerous electrolytic solution.
These fiber-like or pulp-like materials may be made by subjecting polymers that are soluble in or undergo swelling in the nonaqueous electrolyte to melt spinning techniques, wet spinning techniques, flush spinning techniques, or ejection coagulation fiber forming techniques and the like for organic solvent solutions of polymers.
Further, pulp-like material, monofilament-like material, side-by-side type composite fiber, true conjugate composite fiber, sea-island type composite fiber, or spliced fiber of composite fibers may be employed in terms of the fiber configuration. As for the composite fibers, there may be used composite fibers of some polymers that are soluble in nonaqueous electrolytic solution or composite fibers of a polymer soluble in nonaqueous electrolytic solution and a polymer insoluble in nonaqueous electrolytic solution.
Polyacrylonitrile, polymethacrylonitrile, acrylonitrile/vinyl acetate copolymers, acrylonitrile/(meta)acrylate copolymers, acrylonitrile/vinyl chloride copolymers, acrylonitrile/methacrylonitrile copolymers, polymethyl methacrylate, vinylidene fluoride/hexafluoro propylene copolymers, polyethylene oxide, polypropylene oxide, ethylene oxide/propylene oxide block copolymer, polyvinylacetate, and acetylated cellulose may be given as polymers to be used in the practice of that are soluble in or have swelling properties under the nonaqueous electrolytic solution. Among these polymers, polymers comprising 50% or more by weight of polymer units of acrylonitrile preferably 60% or more by weight, may be preferable from the viewpoint of handling characteristics, ability to carry and retain the nonaqueous electrolytic solution, and gel-forming ability.
In case of a fibrous sheet-like material to be used in the practice of the present invention, there may be used a pulp-like or short fiber-like material comprising a polymer soluble in a nonaqueous electrolytic solution, a non-woven fabric or paper-like sheet formed by adding thereto fibers or pulp made of polymers that are insoluble in a nonaqueous electrolytic solution, or a material obtained by attaching by mutual entanglement the above short fiber-like or pulp-like material into a woven structure of woven material made from fiber comprising polymers which are insoluble in a nonaqueous electrolytic solution.
Natural pulp, polyethelene pulp, aromatic polyamide pulp, polypropylene pulp or polyolefine fiber, polyfluro vinylidene based fiber, aromatic polyamide fiber, polyphenylene sulfide fiber, polysulfone fiber, ceramic fiber, or glass fiber, etc., may be used for the fiber or pulp which is insoluble in the nonaqueous electrolytic solution when the fibrous sheet-like material is formed to be used in the present invention.
It is preferable for the fibrous sheet used in the practice of the present invention to include 50% by weight or more of fiber-like or pulp-like material comprising polymers soluble in or swelling by the nonaqueous electrolytic solution. Fibers or pulp comprising polymers which are insoluble in the nonaqueous electrolytic solution or natural materials, the fibers or pulp being included in the fibrous sheet-like material are components greatly contributing in the increase of the strength of a gel electrolyte sheet made using this fibrous sheet-like material.
The porosity of the fibrous sheet-like material to be employed in practicing the present invention is preferably in the range of 20 to 80% by volume, because the impregnation characteristic of the fiber sheet-like material by the nonaqueous electrolytic solution is improved, and whereby a nonaqueous electrolytic polymer gel layer with uniform characteristics is formed.
As for examples of the nonaqueous electrolytic solution used in the practice of the present invention, there may be found aprotic polar solvents with a high dielectric constant or a low viscosity, or organic solvents or mixtures thereof which are electrochemically stable and which will readily dissolve the electrolyte salts described below. Examples of the above may be found in; carbonates, such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate (EMC), etc.; lactones, such as xcex3-buytylolactone (BL), amide based solvents, such as dimethyl formamide (DMF), dimethyl acetimide (DMAC), etc.; and sulfolane, acetonitrile, dimethyl sulfoxide (DMSO), tetrahydrofuran, or dimethoxyethane and the like. Examples of electrolyte salts may preferably found in quaternary alkylammonium salts as an electrolyte in a nonaqueous solution, and as a specific example such salt having the structure (R4N)+(M)xe2x88x92 may be used. Examples of alkyl groups, represented by R, may be found in methyl, ethyl and butyl groups and the like, and examples of the anion component M may be found in tetrafluoroborate (BF4), and perchlorate (C104) groups and the like
The electric double layer capacitor of the present invention is made by the steps of housing the fibrous sheet-like material of the present invention in a container while the fibrous sheet-like material is being sandwiched between polarizable electrodes, injecting the container with nonaqueous electrolytic solution, preferably at reduced pressure thereby causing the fibrous sheet-like material to be sufficiently impregnated by the nonaquerous electrolytic solution, and causing at least one part of the fibrous sheet-like material to be dissolved or swelled, followed by causing the part to form a gel.
The lamination of the polarizable electrodes and the fibrous sheet-like material for forming a polymer gel electrolyte layer can be handled with these raw materials in a noncohesive state, and thus the rate of occurrence of defects in the process of assembling the capacitor can be kept low.
The impregnation of the nonaqueous electrolytic solution into the fibrous sheet-like material can utilize the capillary phenomenon based on the fiber structure, and thus, the nonaqueous electrolytic solution rapidly impregnates the fibrous sheet-like material, and there occurs almost no area of unimpregnation, thereby making it possible to provide capacitor equipped with a polymer gel electrolyte layer with almost no microvoids.